1. Technical Field of the Invention
This invention relates to devices having rotating shafts supported by end bearings, and in particular to scanning galvanometers and other reciprocating rotor devices wherein rotor acceleration contributes to repetitive asymmetrical loading on the rotor bearings.
2. Background Art
The rotor and load of a galvanometer scanner have mass, and as a result act like a gyroscope rotor when rotating. Because of this, the rotor-load assembly resists any attempt to change the axis of rotation, and also any attempt to change its speed of rotation. This phenomenon is recognized in the motions of a toy top, which stands upright on it""s tip when first released, and then begins to tip it""s axis of rotation with respect to gravity and to precess that axis as friction with the air and between the tip and the surface upon which it rests gradually slow down the spin.
Galvanometer scanners are not often exposed to forces which attempt to change the tilt of the axis, although they can be in some applications on mobile platforms. On the other hand, galvanometer scanners are almost always used in applications in which the speed of rotation is changed rapidly, and in the limit, reversed in sense. In fact, it is one of the attributes sought in galvanometer scanners that they be capable of extremely rapid changes in rotational speed and direction. For example, it is common to have a beam scanning galvanometer with a rotor motion that is represented as continuous saw-tooth waveform in which one cycle consists of a constant velocity period of movement in one direction for xe2x80x9cscanningxe2x80x9d, followed by a rapid fly-back period of slowing and reversing of direction, motion in the other direction, slowing and reversing direction again, and re-acceleration to scan speed. Any of these changes results in an attempt by the rotor and its load to precess, and the attendant moments are resisted by the bearings which support the rotor and load.
Examples that may provide context for the reader include the prior art of Montagu""s U.S. Pat. No. 5,225,770, which illustrates a conventional rotor and bearing arrangement. The prior art of Chandler""s U.S. Pat. No. 5,280,377 provides a contextual explanation of the forces to which a scanning galvanometer rotor of this sort is subjected, including the acceleration forces occurring during the fly back period between constant speed forward scan.
In addition, any imbalance in the rotating parts causes the rotor assembly to attempt to rotate on an axis which passes through the mass center, and to the degree that this axis departs geometrically from the rotor axis, a set of additional moments is imposed periodically on the bearings. As a practical matter, some degree of imbalance is always present in a working galvanometer scanner.
In the limit, these moments can exceed the load rating of the bearings, and cause irreversible damage to the bearing parts, in particular the raceways and balls, which leads to loss of smoothness of operation and eventually to failure of the bearing.
Since these issues are not confined to galvanometer scanners, and are characteristic of all rotating machinery, much effort has been devoted to ameliorating the effects of the gyroscopic moments on bearings. Modern rotating machinery, like galvanometer scanners, is balanced to the highest degree of precision practicable. Bearings have been developed to display very great resistance to what is called in the trade xe2x80x9cBrinellingxe2x80x9d, a name for the localized dents in the balls and rings which results from exceeding the elastic limit of the materials. Special materials, such as ceramics, which have very high elastic limits in compression, have been used. However, galvanometer designers continue to produce designs which are capable of greater accelerations than the bearings can stand.
Moving for background purposes to an unrelated field of art; rheology is defined as the science of the flow and deformation of matter. Some materials approach the behavior of ideal fluids and are described as being viscous. Other materials approach the behavior of ideal solids and are described as being elastic. Visco-elastic materials may be formed into fabricated useful shapes having desirable rheological properties.
Load shock and vibration from other sources also cause potentially damaging asymmetrical bearing loads. Devices for dampening irregular, externally induced load shock and vibration in rotating assemblies, such as road surface effects in vehicle axle bearing assemblies, have been the subject of patents. By way of example, Pinkos et al""s U.S. Pat. No. 5,730,531 discloses a center bearing assembly with rheological fluid for dampening vibrations. An electromagnetic field, varied by an electronic controller in relation to the axle speed, acts on the rheological fluid in the dampener, to increase or decrease the effectiveness of the dampener as between the center bearing and the support structure.
In another vibration dampening application, Duggan""s U.S. Pat. No. RE36270, reissue of U.S. Pat. No. 5,452,957, first published in Sep. 26, 1995, discloses a vehicle axle and bearing assembly, with the axle and center bearing supported within a donut-shaped bladder formed of an elastomeric material and filled with a rheological fluid, the assembly being attached by a suitable support bracket to the vehicle. A controllable source of electromagnetic field is located adjacent the bladder, and used to vary the flow or shear characteristics of the rheological fluid in the bladder, thus offering a variable dampening capability.
The rheology art described above has no where been suggested as useful for or applicable to the general art and the particular shortcomings of high speed reciprocating rotor and galvanometer bearings as to the problem of out of balance conditions and asymmetrical bearing loads and damage caused by acceleration.
As was intimated in background section, in the special case of galvanometers scanners and other similar high speed reciprocating rotor devices, what is often desired is a scan or rotary position versus time profile which might, for example, be a saw-tooth waveform in which there is a constant velocity period followed by a rapid fly-back period, similar to the horizontal scan waveform of a television tube. Of course, in the television tube, it is an electron beam, rather than a rotor of significant mass, that is being manipulated.
Just as in the case of the television tube, the part of the scan which is required to be precise is the constant velocity xe2x80x9cforwardxe2x80x9d portion. The fly back portion is relatively uncontrolled, but is required to take place in the shortest possible time. As a result, a very large acceleration is applied at the end of the forward scan to slow and reverse the direction of rotation, return the rotor to the other end of its angular path, slow and reverse its direction again, and re-accelerate it to scan speed. The degree of difference in the asymmetrical loading between the scan phase and the fly back phase is substantially the result of rotor acceleration. It is the gyroscopic loads induced by these accelerations which have the most potential for damage to the bearings, but which occur during the part of the scan which is not required to be under precise geometrical control.
This observation leads to the possibility of providing a coupling between the bearings and their housing which is stiff during the constant-velocity portion of the scan, assuring adequate geometrical precision, but is resilient during the period of large bearing moments. This coupling would desirably absorb some or most of the force applied between the bearing and it""s housing during rotor acceleration by deflecting, but would return to it""s initial position when the acceleration forces disappeared.
In pursuit of the goals of the invention, such a coupling has been successfully constructed by placing a visco-elastomeric xe2x80x9cOxe2x80x9d ring or a multiplicity of xe2x80x9cOxe2x80x9d rings in a concentric fashion between the outer raceway of each bearing and the bearing housing. The composition of the xe2x80x9cOxe2x80x9d ring material is selected to provide the best absolute stiffness, relating and affecting the necessary rigidity of the rotor alignment to assure precision during the constant speed forward scan. Spacers between the xe2x80x9cOxe2x80x9d rings control the degree of compression of the elastomer rings, and thus their relative stiffness, during the fly back periods of relatively high asymmetrical loading.
Many elastomers display non-linear rheological effects. These materials, like glass, are not really solids. They can be thought of as super-cooled fluids. As a result, they flow slowly in the presence of even very small forces. The most often encountered and recognizable consumer product example of these materials is Silly-Putty(trademark) bulk play material, which is a silicon-based, visco-elastic, non-Newtonian fluid constructed to display the unusual attributes of these materials in a most graphic way. (No claim is made to the trademark, Silly-Putty(trademark).) For the adults among us, the modern golf ball is another example.
A way of visualizing the mechanism is this. The molecules making up the material are very high molecular weight polymers whose structure is asymmetric, so that the molecules twist themselves up into tight spirals. The tightness of the spiral is a measure of the energy contained in the molecule. As a result, these spirals are quite temperature sensitive.
In the case of rubber, when you heat a rubber band, that is, add energy to the molecules, the spirals get tighter, and the rubber band shrinks in length, displaying what appears to be a negative temperature coefficient of expansion. Other examples of this molecular twisting are the reaction of spiro-paran dyes to exposure to light of the correct wavelength. These dyes, commonly used in welder""s face shield lenses, capture the UV energy from the arc and tighten up their twist so that they are no longer transparent to the harmful radiation emitted by the arc. When the arc is quenched, the dye looses the stored energy and un-twists, again becoming transparent. This process is so fast that the welder is not conscious of the initial flash of the arc.
Of course, in the instant application, neither heat nor light play an essential part in the material""s response to changes in loading. Instead, the mechanical forces do the work. At first, the material displays high stiffness (assuming it has been optimized for the application), because the molecular xe2x80x9cspringsxe2x80x9d are tightly twisted, and the spirals interfere with each other. This is the condition during the forward scan. Under the high loading forces produced by acceleration, the springs eventually stretch enough so that the coils interfere less and less, and the molecules begin to slide past each other. This is the non-linear low-stiffness phase. When the forces are reduced at the end of the acceleration period, the material coils itself back into the high-stiffness phase.
In general, the stiffness curve has the shape of an inverted hockey stick. The initial relatively flat portion represents the area in which the material acts like a bunch of tiny springs in parallel, the joint between the blade and the handle the transition region, and the high-negative-slope handle represents the area in which the almost-straightened-out molecules slide past each other relatively easily.
In accordance with the invention, the appropriate stiffness characteristics of the xe2x80x9cOxe2x80x9d ring are established in each application as a compromise between the degree of geometrical precision desired during the high-precision forward scan, and the degree of compliance required to absorb the shock-like forces during the fly-back. Both the selection of the visco-elastomeric material, and the geometry of the these elements and of the bearing assembly as a whole, control the initial spring constant, the transitional force, the time rate of change in the transition area of the curve, and the rate of change of the slope in the transition area of the curve. In summary, this novel methodology of introducing a rheological bearing coupler (RBC) into the bearing assembly of galvanometer scanners used in high duty cycle applications, and in other devices employing reciprocating rotors in a similar fashion, to relieve bearing stresses during periods of acceleration, results in a significant increase in the life expectancy of the bearings.
Other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein we have shown and described only a preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by us on carrying out our invention.